Does it really matter?

Do you have an issue with other non-native in the game, like the Mook, the Helazoid (native to Lost Guardia, albeit related to the Dominus-native Higardi), or the races available for player created characters?

It occurs to me that "destroy all non-natives" could be a possible challenge, though a tricky one, mainly because of some complications:
* The races allowed for PCs are not native, so you need every one of them to be dead by endgame. This means you *have* to use RPCs, and only some of them count as native.
* RFS-81 is not native (unless you consider the Dark Savant to be native, in which case you'd want to take the evil ending). I'd count Vi Domina as native (though, unlike the other natives, she's traveled to other planets, including Lost Guardia), but unfortunately she leaves the party when the final battle starts. I don't know if there's any other native RPC who will willingly go to Ascension Peak.
* One other constraint I'd add: You must get one of the 3 real endings, not the one that results in Dominus being destroyed, with you on it.

My issue is not with non-nativity (as an all-Human party, I am a foreigner to Dominus maybe, if you don't count Humans as Higardi).

It's more of a live-and-let-live philosophy. The Brotherhood of the Ascension Monks, Higardi, Trynnie, Rapax, and Rnyjin, are native to Dominus. Let them do their own thing (the first 3 are Friendly to the party, the last 2, not so much, though you can make friends with the Rapax Common and Templar by cozying up to Al-Sedexus).

The foreign races, I have no problem with either. The Mook are probably the most gentle, scientific race in the game. The Umpani and T'Rang are warring factions; Why not make friends with both, as neither have a problem with the party, and wish to gain the party's support? The Helazoid faction isn't really in this game, until you go to Bayjin, and see the ship shot down by the Dark Savant, and witness the end of Jan-Ette.

BUT NO. The Rattkin, the foreign race, that takes over the Trynnie's Sixth Bough. Just no. An evil Rattkin Mafia. Slaughter them all.

Sorry that I went off on a tangent about the Rattkin. The original post was supposed to inquire if hybrids (Lord, Valkyrie, Ranger, Ninja, Monk, Samurai), should get Powercast. The answer seems to be a definitive no, as it prevents them from maxing out their melee and ranged combat abilities (they are not full casters, suffer a -4 caster level penalty, and Intelligence is almost worthless by itself).

The only use for Powercast, would probably be for full casters, who don't suffer the -4 caster levels. They tend to be physically weak, and should at least go for magic mastery, as their physical abilities tend to be low.